1. Field of the Invention
The present invention relates to an electroluminescent display (hereinafter referring to as “ELD”), and more particularly, to an organic ELD display having a dual-plate structure.
2. Background of the Related Art
Recently, a lot of effort has been made to develop flat displays, such as liquid crystal displays (LCDs), field emission displays (FEDs), electroluminescent displays (ELDs), plasma display panels (PDPs), and the like. Among these flat displays, the LCDs have been widely used in personal information devices, such as personal communication service terminals, laptop computers, TVs, and so on. Since the LCD has disadvantages of a narrow viewing angle and slow response time, a self-luminescent organic ELD has attracted considerable attention.
The organic ELD device utilizes an electroluminescence phenomena, in which an electric field is applied to a negative electrode and a positive electrode formed in the upper and lower sides of an organic emitting layer such that electrons and holes are inserted into and transported into the organic emitting layer so that the electrons and holes are coupled with each other and energy generated when the electrons are coupled with the holes is emitted in the form of visible light. In other words, the electrons and the holes form hole-electron pairs, and the hole-electron pairs emit light when dropping from the excited state to the ground state.
The organic ELD device can be driven at a low voltage and display all colors within a visible spectrum. Also, the organic ELD device can satisfy a variety of consumers' tastes because of its advantages of a fast response time, excellent brightness, and thin films. Moreover, the organic ELD device can be implemented on a flexible transparent substrate, such as a plastic. Since the organic ELD device can be driven under the low voltage, exhibit relatively low power consumption and easily display three colors (green, red, and blue colors), it is ideal for use as the next generation flat display.
The organic ELD device is either a passive matrix type or an active matrix type depending on a driving method thereof. The passive matrix type organic ELD device has a simple structure and therefore, a manufacturing method thereof is also simple. However, the organic ELD device has high power consumption, and has limitations in achieving a large sized organic ELD device. Moreover, the more the number of lines, the lower the aperture ratio. On the other hand, the active matrix type organic ELD device has a high luminescent efficiency and provides a high-density resolution. Hereinafter, an active matrix type organic ELD device having a dual plate structure according to the related art will be described in detail with reference to the accompanying drawings.
FIG. 1 is a cross-sectional view illustrating an organic ELD device according to the related art, and FIG. 2 is a cross-sectional view highlighting problems of the organic ELD device of FIG. 1. As shown in FIG. 1, the related art organic ELD device is fabricated by bonding a first transparent substrate 10 and a second transparent substrate 20 together using a sealant (not shown). A plurality of pixels (luminescent parts) P are defined on the first substrate 10, and thin film transistors T and array lines (not shown) are deposited at respective sides of the respective pixels P. Specifically, on the first substrate 10, gate lines are arranged in a single row, data lines and power lines cross each other and are spaced apart from each other by a predetermined interval, switching thin film transistors Ts (not shown) are provided at positions where the gate lines cross the data lines, and driving thin film transistors Tp are provided at positions where the gate lines cross the power lines. Moreover, the switching thin film transistors Ts (not shown) have drain electrodes connected to gate electrodes 12a of the driving thin film transistors Tp. The driving thin film transistors Tp have drain electrodes 15b integrally formed with connection electrodes 41. A set of the gate line, the data line, and the power line defines a sub-pixel, and respective ones of the switching thin film transistors Ts and the driving thin film transistors Tp are provided in the sub-pixel.
On the second substrate 20, first electrodes 21 are formed as transparent hole injection electrodes, insulation layers 31 and separators 32 are disposed at the boundaries of the sub-pixels, organic luminescent layers 22 are formed on the first electrodes 21 between the insulation layers 31 and the separators 32, and second electrodes 23 are formed as electron injection electrodes on the organic luminescent layers 22. The organic luminescent layers 22 and the second electrodes 23 are each formed with separate patterns divided in a unit of the sub-pixel by the insulation layers 31 and the separators 32. The second electrodes 23 are connected to the driving thin film transistors Tp through the connection electrodes 41. Each of the connection electrodes 41 is deposited on the first substrate 10, contacting the second electrode 23 when the first substrate 10 is bonded to the second substrate 20. The connection electrodes 41 contact the second electrodes 23 through contact spacers 42, each of which is formed with a column-shape and a predetermined height between the two substrates to maintain a cell gap.
The organic luminescent layer 22 emits light with colors of red R, green G, and blue B. Generally, separate organic materials for emitting red, green, and blue are patterned in respective pixels. The organic luminescent layer 22 may be either a single layer or multiple layers. If formed with the multiple layers, the organic luminescent layer 22 further includes hole-injection layers and hole-transporting layers disposed near the first electrodes 21, and electron-injection layers and electron-transporting layers disposed near the second electrodes 23. When an electric field is applied to the first electrodes 21 and the second electrodes 23 of the organic ELD device, electrons are injected into the organic luminescent layer 22 from the second electrodes 23, and holes are injected into the organic luminescent layer 22 from the first electrodes 21.
However, the related art organic ELD device and method for fabricating the same has some problems. The related art organic ELD device has the single contact spacer 42 in the sub-pixel, and the connection electrode 41 is formed on the contact spacer 42. As shown in FIG. 2, when patterns 60 (for example, power lines) are further formed under the contact spacer 42 or the insulation layers 61 have uneven thickness, a cell gap of the organic ELD device is uneven. For this reason, when the cell gap in the sub-pixels is uneven, there occurs a problem, indicated by “A” in the drawing, that the connection electrode 41 on the contact spacer 42 does not contact the second electrode 23. In such a situation, the organic luminescent layer cannot be driven.